Yah, this is kind of too broad of a question to answer specifically, but keeping your hips low is a good idea, as Ramirez pointed out. With that being said, being thrown by a tai otoshi requires a lot different defense that say against an attacking Osoto Gari. Taking up the space and keeping your weight back will defend the tai, but get you killed against the Osoto.

Ran across this on Judo info, and it might be worth a look, or at least prompt some questions that you can delve into with your coach.